1. Field of the Invention
The invention relates to a method and apparatus for tuning an active filter, more particularly to a method and apparatus for tuning the 3-dB corner frequency of filters to approach a constant characteristic.
2. Description of the Related Art
As development of integrated circuitry technology is accelerated, necessary functions are integrated within a single chip. In particular, analog filter circuits implemented by capacitors and resistors are widely used in electronics or communication products. In the design and manufacture of active continuous-time filter, the frequency response is directly proportional to variation of the values of resistors and capacitors. As is well known in the art, the use of capacitors and resistors generates RC-product shifts on account of variations in temperature, supply voltage, and manufacturing process. Unavoidably, variation in manufacturing process and variations during operation cause resistance of a resistor with approximately ±21% deviation, and capacitance of capacitor with approximately ±10% deviation. In other words, active filters result in RC time constant deviations from their design value up to ±32%. As a result, tuning circuits may conventionally be used with analog filter circuits in order to fine tune or adjust the filter to compensate for variation in the analog components of the filter.
The employment of integrated active filter circuits in combination with external high precision resistors and capacitors to compensate for the above-mentioned variations is a solution to such problem. However, this solution conflicts with the advantages offered by integrated circuits, such as low cost and small form-factor (few or none external components) of the filter circuit. Therefore, it has become increasingly common to embed an automatic tuning circuit as a part of a chip to calibrate the RC time constant deviation.
Traditionally, calibration of RC time constant is based on two invariant identities to temperature and processing, bandgap voltage and a clock frequency. One way to achieve a tunable RC time constant is to provide active resistors, i.e. resistors fabricated as MOSFETs instead of passive resistor elements, and control the MOSFET to provide a desired resistance. In such an arrangement, a feedback circuit measures the actual RC time constant of the filter with reference to a clock frequency, and provides a corresponding signal to the MOSFET to continuously adjust their resistance to attain the required time constant. This solution, however, needs a continuous input signal for the MOSFET and thus causes an increase of power consumption of the filter circuit. Moreover, this approach is disadvantageous when a low supply voltage is used (e.g. as low as 1 V); since the MOSFET, in general, requires a large sub-1V threshold voltage to be conductive, such that the MOSFET cannot provide a sufficient variable control range to compensate for the large variations of the active filter.
As a result, in order to solve such problem, the need for an improvement on the method and apparatus for tuning an active filter is required.